Free piston linear generator

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The free-piston linear generator (also FPLG shortly FKLG , Engl. Free piston linear generator , FPLC) is a free-piston engine , researchers are working at the stand with 2016 and developed. The machine is used to convert the energy chemically bound in a fuel into electrical energy.

Basic principle of the free-piston linear generator (schematic representation by DLR)
Functional demonstrator of the free-piston linear generator for the proof of concept (test setup at DLR in Stuttgart)

Conceptual classification and delimitation

The free piston linear generator belongs to the group of free piston generators (also "free piston motors with electrical energy extraction"), which in turn falls into the class of free piston machines. The concept of the free piston linear generator has been coined since the early 2000s in particular by the German Aerospace Center (DLR) , which carries out a large proportion of the corresponding research and development work in German-speaking countries. According to the use of the term by the DLR, a free-piston machine is called a free-piston linear generator if it has the following subsystems:

  1. A combustion chamber (or several) with internal combustion in a single or opposed piston design
  2. One electric linear generator (or several)
  3. A return spring unit (or several), typically designed as a gas spring.

technology

Structure and functionality

The free piston linear generator consists of the three subsystems combustion part, linear generator and gas spring. These are coupled to one another via a rigid piston-rotor unit. This piston unit swings between two gas cushions that build up in the combustion section or in the gas spring. A fuel-air mixture is ignited in the combustion cylinder, so that the pressure increases and the piston unit accelerates in the direction of the gas spring. This compresses the gas in the gas spring cylinder so that the pressure in the gas spring cylinder increases. The piston unit is first decelerated and then accelerated back towards the combustion cylinder. During the reversal process, the gas exchange takes place in the combustion cylinder, i. H. the exhaust gas is displaced by fresh charging. Then the process begins all over again. During each movement of the piston unit, the linear generator takes kinetic energy from the system and converts it into electrical energy. So that this can happen both during expansion and compression (on the part of the combustion part), part of the energy is temporarily stored as potential energy in the gas spring.

variants

For most applications, in particular applications in motor vehicles, two opposing piston units are to be used for reasons of mass balancing and to be synchronized via a corresponding control. In the simplest case, a second unit is built up in mirror image, which has no functional connection to the first. Alternatively, the functional spaces of the two gas springs or the two combustion parts can be merged. In the case of two piston units with a common combustion part, an opposing piston system is created, which in turn limits the possibilities for gas exchange. Instead of valve control, only slot control is then possible.

Furthermore, alternative arrangements of the overall system result from variation of the linear generator. This can be made round or flat, for example.

With regard to the combustion part, both operation in the two-stroke process and in the four-stroke process is basically conceivable. The latter, however, requires a significantly higher effort for the intermediate storage of the energy after the work cycle or for the acceleration of the piston unit in the “suction” and “compression” cycles. The two-stroke process is therefore favored in current research projects. Several variants are also conceivable with regard to the combustion process:

  • Spark ignition ( Otto )
  • Compression ignition ( diesel )
  • Homogeneous Compression Ignition (" HCCI ")
  • Free piston linear generators with external combustion (linear Stirling engine) are known.

Potential

In various technical domains, the free-piston linear generator has principle-related advantages over conventional technologies (internal combustion engine or internal combustion engine-generator unit). Some of these have already been implemented experimentally, while the practical implementation and also the quantification of the potential is the subject of current research and development work. The following are listed as the most important potentials:

  • High efficiency / reduced fuel consumption
    • In connection with low pollutant and CO 2 emissions, especially at partial load
    • Through optimized combustion with variable compression, reduced expansion and variable piston movement
    • By reducing friction (fewer moving parts, elimination of lateral piston forces)
    • Through special combustion processes (especially HCCI)
    • Numerical values ​​in a sample design by DLR: 36.6% for the overall system and 43.0% indexed
  • measures and weight
    • Flat design with the option of mounting in the underbody of a car
    • Good power density and low weight (advantages of 5 to 15% in a sample design by DLR)
    • Proportions can be flexibly designed in the course of development
  • Fuel variability
    • Through variable compression
    • Switching between different fuels in operation
    • Uncomplicated (mostly software-based) adaptation to different fuels in the context of development and production
    • Optimal use of the full knock resistance of each fuel
  • Quiet and low-vibration operation
    • By ideally balancing all inertial forces of all orders, when using a system with two opposing piston units

state of research

Free piston engines that meet the above criteria in their structure are known under various names from several research and development projects:

  • Jarret, France, 1971
  • Free Piston Engine, Van Blarigan, Sandia National Laboratory, since 1995
  • Free-Piston Engine Project, Sir Joseph Swan Institute for Energy Research, Newcastle, UK, 1999
  • Free piston linear generator, DLR, since 2002
  • Internal Combustion Linear Generator Integrated power System, Xu, Nanjing, China, 2010
  • Active Crank Train Free Piston Engine, University of Lincoln / Lotus Engineering, GB, 2012
  • The BEETRON project, micromer ag, Switzerland, 2012
  • Free Piston Engine Linear Generator "FPEG", Toyota Central R&D, Japan, 2014
  • Aquarius engine-generator, Aquarius Engines, Israel, 2016

In February 2013, DLR provided the proof of concept ( proof of concept ) on its test benches in Stuttgart for the first time worldwide . For the first time, a system of the construction method under consideration was successfully put into operation. In the first tests, an electrical power of 8 kW could be taken from the single piston system. Toyota Central R&D also published the commissioning of the first "FPEG" in April 2014, whereas the majority of the other worldwide projects were limited to simulations or individual hardware aspects and could not commission an entire system. In 2016, the Israeli startup Aquarius Engines announced that its range extender engine based on a single-cylinder free-piston engine would be tested by PSA Peugeot CITROËN.

Applications

Micro-CHP units

A free-piston Stirling engine with linear generator has been available from Microgen since 2010 . From 2011 he was in block heat and power plants , u. a. installed by the companies Brötje , Senertec and Ökofen.

Future application scenarios

Future application scenarios cover the following areas, for example:

  • Driving road vehicles (cars, trucks, buses, etc.). The entire powertrain topology is referred to as a “Range Extended Electric Vehicle (REEV)”, “Serial Hybrid Vehicle” (SHEV) or “Free Piston Electric Vehicle” (FPEV). The free-piston linear generator generates all or part of the electrical energy that is required to supply the electrical traction motors.
  • Drive of rail vehicles. The way the drive train works corresponds to the diesel-electric locomotives that are widespread today , with the internal combustion engine and generator being replaced by one or more free-piston linear generators.
  • Propulsion of ships. The mode of operation of the drive corresponds to that of the known diesel-electric ship drives, with the internal combustion engine and generator being replaced by one or more free-piston linear generators.
  • Stationary energy supply, in particular to provide control energy and cover peak loads
  • Combined heat and power in combined heat and power plants
  • Mobile and stationary (emergency) power supply, for example in hospitals, disaster areas or at major events
  • Auxiliary power supply (engl. Auxiliary power unit , APU) for commercial aircraft or truck

Individual evidence

  1. ↑ Sven-Erik Pohl: The free piston linear generator - Theoretical considerations of the overall system and experimental investigations on the partial system of the gas spring , Helmut Schmidt University, Hamburg, 2007
  2. ↑ Cornelius Ferrari: Development and investigation of a free-piston linear generator with special consideration of the internal combustion engine subsystem with the help of a new, fully variable test bench , University of Stuttgart, Stuttgart 2012
  3. ↑ Specifications Microgen Unit 1 kW
  4. ↑ M. Razali Hanipah, R. Mikalsen, AP Roskilly: Recent commercial free-piston engine developments for automotive applications. In: Applied Thermal Engineering . Volume 75, 2015, pp. 493-503, doi : 10.1016 / j.applthermaleng.2014.09.039 . (Review)
  5. ↑ a b c d Florian Kock, Alex Heron, Frank Rinderknecht, Horst E. Friedrich: The free piston linear generator - potentials and challenges , Motortechnische Zeitschrift mtz, 10/2013
  6. ↑ Stephan Schneider, Frank Rinderknecht, Horst E. Friedrich: [ Design of Future Concepts and Variants of the Free Piston Linear Generator ], Ninth International Conference on Ecological Vehicles and Renewable Energies (EVER), 2014
  7. ↑ Florian Kock: Control and regulation of the free piston linear generator - development method and control concept for the operation of a new type of energy converter , University of Stuttgart, Stuttgart 2015
  8. ↑ R. Mikalsen, AP Roskilly: A review of free-piston engine history and applications. In: Applied Thermal Engineering . Volume 2714-15, 2007, pp. 2339-2352, doi : 10.1016 / j.applthermaleng.2007.03.015 . (Review)
  9. ^ A b H. Kosaka, T. Akita, K. Moriya, S. Goto et al .: Development of Free Piston Engine Linear Generator System Part 1 - Investigation of Fundamental Characteristics , SAE World Congress 2014, SAE World Congress 2014
  10. ↑ Denise Nüssle, Horst E. Friedrich: DLR researchers present new range extenders for electric cars , DLR Press Portal, news from February 19, 2013, accessed on May 8, 2015
  11. ↑ Peugeot tests Israeli range-extender technology in electric car push , Reuters, news from July 13, 2016
  12. ↑ https://www.microgen-engine.com/products/engines/
  13. ↑ http://senertec-kell.de/index.php/produkte/dachs-sterling
  14. ↑ Pellet boiler with Stirling engine | ÖkoFEN_e. Retrieved May 14, 2020 .
  15. ↑ https://www.bhkw-infothek.de/nachrichten/2474/2011-04-15-ish-senertec-prasentiert-den-dachs-stirling-se/